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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

460

SOAPS

The aqueous acid liquid, separated from the petroleum ether, may be
utilised for the determination of the alkalies (see 4).

4.  Total Alkalies:—This determination may be combined with the
preceding one of the total fat.    For this purpose it is sufficient to decompose
the aqueous solution of 20 grams of the soap with 100 c.c.of normal sulphuric
acid and then to proceed as in 3, care being taken to lose no trace of the
aqueous acid liquid separated from the petroleum ether solution.

This liquid, collected quantitatively in a conical flask, is titrated with
normal potassium hydroxide solution in presence of methyl orange.

The difference between the volume of normal acid added to the soap
solution and the volume of normal alkali necessary to neutralise the remain-
ing free acid, represents the total alkali—existing as hydroxides, carbonates,
silicates, borates and soaps—in the soap. This is expressed as sodium
oxide for hard or powdered soap, and as potassium oxide for soft soap ;
i c.c. normal H2S04 = 0-031 gram of Na20 = 0-047 gram of K20.

5.  Alkalies combined with Fatty Acids.—The alkalies combined
with fatty (or resin) acids, that is, as soaps, are deduced from the acid
number of the total fat obtained as in 3.

The acid number is determined as on p. 374, and the results are expressed
as Na20 for hard or powdered soap and as K20 for soft soap.

6.  Free Alkalies,—The presence of excess of alkali in a soap is detected
(i) by dissolving I part of the soap in 50 parts of 95% alcohol and adding
a few drops of phenolphthalein solution (red coloration), or (2} by dropping
on to a section of the soap, recently cut, a drop of mercuric chloride solution
(yellow coloration) or mercurous nitrate solution (black coloration).

Quantitatively free alkalies as hydroxide and as carbonate are deter-
mined as follows :

(a)  ALKALIES AS HYDROXIDE.   A solution of 10-15 grams of anhydrous
glycerine in 100 c.c. of absolute alcohol is neutralised, if necessary, with a
few drops of alcoholic potash  (phenolphthalein as indicator).    5 grams
of the soap are then dissolved in it and 2-5 c.c. of cold, saturated alcoholic
strontium chloride solution added to precipitate the alkali carbonates, the
free alkalinity being then titrated with standard alcoholic stearic acid solu-
tion in presence of phenolphthalein.1

(b)  ALKALIES AS CARBONATE.   If the soap does not contain silicates,
borates or other alkaline salts, the alkalies as carbonate may be calculated
indirectly or by difference, by subtracting from the total alkali (see 4) the
sum of the alkali as hydroxides and that combined with the fatty acids
(see 5  and 6a), all expressed as Na20 or K20 : this difference, converted
into Na2C03 or K2C03, represents alkalies as carbonates.

When silicates, borates or other alkaline salts are present, the best way
to determine the carbonates is to decompose the soap with dilute sulphuric

1 The alcoholic stearic acid solution is prepared by dissolving about 7 grams of
stearic acid in a litre of absolute alcohol (approximately N/40 solution) and the titer,
i.e., the number of grams of NaOH or KOH corresponding with i c.c., determined by
N/io-potassium hydroxide solution in presence of phenolphthalein. With reference
to this and other determinations, see the paper by G. Gianoli, " On Uniform Methods
of Soap Analysis" (L'Industrie,, 1914, p.^        0*973